How early can you see a fetal heartbeat on a transabdominal ultrasound?

Depending on the: Quality of the machine and on your anatomy, generally about 6 1/2 weeks. It's about 5 weeks and 5-6 days on transvaginal.

6-7 weeks from LMP: Transvaginal ultrasound can detect fetal heartbeat earlier than transabdominal ultrasound (5-6 weeks after lmp). Transabdominal ultrasound varies a bit more primarily due to differences in maternal size. In general, the fetal heartbeat should be visible transabdominally by 7 weeks from the first day of the last menstrual period.